1. Field of the Inventive Concept(s)
The presently disclosed and/or claimed inventive concept(s) relates generally to oligomeric reaction products formed by the depolymerization of polyethylene terephthalate polymers and methods thereof. More specifically, the presently disclosed and/or claimed inventive concept(s) relates to oligomeric reaction products formed by the depolymerization of polyethylene terephthalate polymer obtained from, for example but not by way of limitation, waste products, such as beverage containers made from polyethylene terephthalate (PET). The oligomeric reaction products can, in one embodiment, be used as a starting material for polyurethanes. The presently disclosed and/or claimed inventive concept(s) also relates to processes for producing oligomeric reaction products from the depolymerization of polyethylene terephthalate. More particularly, the presently disclosed and/or claimed inventive concept(s) relates to a process of producing oligomeric reaction products of polyethylene terephthalate capable of controlling the removal of byproducts during the reaction. The presently disclosed and/or claimed inventive concept(s) also relates to ultraviolet curable urethane acrylate and polyethylene terephthalate compositions and methods of making and uses thereof.
2. Background of the Inventive Concept(s)
Plastics currently represent an ever-increasing portion of the mass of municipal solid waste in North American landfills. The conventional opinion regarding the resistance of plastics to degradation has positioned synthetic polymers as threats to the environment. Traditionally, such an environmental predisposition against synthetic polymers has pushed public opinion and lawmaking bodies to reduce their use and application. Although conservation efforts encourage consumers to use synthetic polymers (or products made front synthetic polymers) sparingly, such efforts will never completely eliminate their use in products. As such, landfills have become de facto repositories of high value petroleum products. Considerable energy, technology, and expense were invested into the production of these petroleum products and the disposal of them into landfills (and/or the biodegradation or incineration of the petroleum products) destroys all the value added efforts undertaken to create them.
In order to overcome the destruction of at least a part of the value added to these petroleum products, recycling has been encouraged. Recycling efforts can generally be divided into two types: mechanical and chemical recycling. Mechanical recycling promotes physical operations for washing and size reduction (for separating unwanted materials), and for reprocessing the recycled materials into new products. Although chemical treatments may be used in an effort to enhance the physical properties of the final product, mechanical recycling is mainly a physical process. Chemical recycling entails the use of chemical reactions to break the bonds of polymeric materials into lower molecular weight products ranging from monomers to intermediate oligomeric compounds. Commercial chemical recycling processes convert this plastic waste stream into an important commodity that can be placed within raw material markets.
Polyethylene terephthalate (PET), illustrated in FIG. 1, is a polymer belonging to the generic family of polyesters. PET is typically prepared by the condensation of terephthalic acid (TPA) and ethylene glycol (EG). TPA and EG are routinely derived from oil feedstock. PET is one of the most commonly recycled polymeric materials. In 1995, for example, 3.5×104 tons of PET were recycled in Europe. When pure TPA and EG are heated together they form the reactive monomer bis(hydroxyethyl) terephthalate (‘BHET’) along with a mixture of low molecular weight oligomers. This mélange of small chain products is permitted to further react and excess EG is removed to form high molecular weight PET, as illustrated in FIG. 2. Many companies produce virgin PET globally giving it different trade names. For example, some of the common trade names of commercially available PET include: RYNITE®, MYLAR®, and DACRON® (Du Pont de Nemours and Company Corporation, Wilmington, Del.) and EASTAPAK® (Eastman Chemical Company, Kingsport, Tenn.).
Academic and industrial studies have focused on chemically recycling PET into its monomeric roots of TPA and EG. Such efforts are often complicated by the high energy and extensive effort needed to purify the monomers from the reaction mixture. As such, chemical recycling of PET typically exhausts the advantages of using such a scrap or waste material. Exemplary methods of obtaining monomers of TPA and EG from PET are given in U.S. Pat. Nos. 3,377,519, 3,801,273, and 3,956,088, all of which are hereby incorporated by reference in their entirety. Similarly, U.S. Pat. No. 3,544,622 (the entire contents of which is hereby incorporated by reference) discloses a variation to previously known approaches wherein the reaction is carried out under conditions to produce a water insoluble salt of terephthalic acid which is separated, washed, and thereafter acidified to produce terephthalic acid. Additional patents have also been issued on various improvements to the above-noted processes, such as U.S. Pat. Nos. 5,045,122, 5,223,544, 5,328,982, 5,414,107, 5,532,404, 5,710,315, 6,075,163, 6,255,547, 6,580,005, 6,649,792, 6,723,873, 6,770,680, 7,098,299, 7,173,150, and 7,338,981, the entire contents of each of which are incorporated herein by reference in their entirety.
Popular pathways for chemical recycling of PET include: hydrolysis, methanolysis, and glycolysis, which are generically depicted in FIG. 3. In FIG. 3, 310 represents a generic polyethylene terephthalate chain of typical size with R1 being a non-hydrogen molecule; 320 represents a nucleophile intended to serve as a model molecule that can attack the ester carbonyl freely (identified as a strong nucleophile in this example as it would bear a charge), which can be generic in structure or species and may or may not be organic in nature, and wherein R2 can be hydrogenic (for hydrolysis), methyl (for methanolysis), or ethyl hydroxyl (for glycolysis); 330 represents the quaternary transition state after the nucleophile has attacked the carbonyl carbon and before the leaving group departs; 340 represents the new ester formed after the leaving group departs; and 350 represents the leaving group. These pathways all utilize transesterification to drive the depolymerization of PET. The extent of the depolymerization generally determines the value of the products formed. Interest in hydrolysis, for example, stems from its ability to provide a direct route to TPA and EG. Unfortunately, hydrolysis suffers from long reaction times at higher reaction temperatures and pressures as well as high costs associated with the purification and separation of the recycled TPA and EG. Hydrolysis of PET can be carried out in basic, acidic, or neutral conditions. Acidic and basic conditions promote an ester carbonyl attack that results in transesterification and the replacement of the organic alkoxide with a hydroxide. Neutral hydrolysis can be performed with a variety of well-studied Lewis acid metal cations, for example.
Methanolysis processes depolymerize PET with methanol at high temperature and pressure. The reaction products of PET methanolysis are dimethyl terephthalate (DMT) and EG, which can then be used as the raw materials to produce PET polymer. Methanolysis employs soluble catalysts (e.g., zinc acetate, magnesium acetate, cobalt acetate, etc.) to improve the reaction rate. As the polymer is broken into more simplified components, ethylene glycol is released. Recombination will rapidly begin if the catalyst, methanol, and DMT are not separated. DMT is typically obtained as a post reaction precipitate after cooling. The driving feature for methanolysis is the insertion of an alkoxide into the ester via transesterification.
Glycolysis promotes the depolymerization of PET using organic dialcohols along with transesterification catalysts to break the ester linkages and replace them with hydroxyl terminals. Preferred agents for such depolymerization are EG, diethylene glycol (DEG), and propylene glycol (PG). Such glycolysis agents can be recycled ethylene glycol, recycled diethylene glycol, and recycled propylene glycol, recycled neopentyl glycol, and combinations thereof. Glycolysis is conducted in a wide range of temperatures (e.g., 150-250° C.) and for a reaction period of from 0.5-8 hours. Usually, 0.5-2% by weight of catalyst (e.g., zinc acetate) in relation to the PET content is added.
The prior art generally teaches the depolymerization of PET by glycolysis in which the PET is depolymerized all the way to, almost exclusively, bis(hydroxyethyl) terephthalate (BHET), which requires an enormous amount of energy. See, e.g., U.S. Pat. Nos. 4,609,680, 5,559,159, and 5,635,584, British Pat. No. 610,136, and Japanese Pat. Pub. No. 61447/1973, the entire contents of each of which are incorporated by reference in their entirety. Additionally, such methods taught in the prior art have significant process limitations resulting from the unwanted solidification of reactants during the reaction processes (which interferes with the agitation of the system) and the formation of unwanted byproducts (which significantly interfere with the critical reaction temperature). Of particular significance is the formation of byproducts, which are generally produced by the glycol(s) reacting with itself and/or the catalyst(s). In one embodiment, the presently disclosed and/or claimed inventive concept(s) is directed to a process for depolymerizing PET into a blend of oligomers rather than a majority of BHET, which greatly reduces the energy necessary for recycling PET. Additionally, in another embodiment, the presently disclosed and/or claimed inventive concept(s) is directed to a process capable of controlling the removal of byproducts and other impurities formed during the depolymerization of PET, which shortens the reaction time while ensuring that the depolymerization reaction goes to completion and that the blend of oligomers produced are of a suitable product quality.
When constructing polymers, polyols are often used to enhance structural behavior and performance. Polyols are compounds with multiple hydroxyl groups available as nucleophiles for chemical reactions. Polyols can take on several shapes and sizes. From small molecules (e.g., glycerin) to larger and more complex molecules (e.g., sucrose). Polyols are primarily used as the starting point for many polymeric systems. Additionally, they can be reacted with propylene or ethylene oxide, for example, and made into polymers or large oligomers themselves. Such “self-made” polymers can thereafter be further reacted and/or combined with a wide variety of reactive moieties to form polymers of increasing complexity or specificity. In addition to being classified as either a polyether or a polyester, polyols can be further delineated according to their structure/application as either flexible or rigid. Such physical characteristics come from the particular polyol's functional moieties and molecular weight. Holding all else equal, flexible (SOFT) polyols have molecular weights from 2,000 to 10,000, and rigid (HARD) polyols have molecular weights from 250 to 700.
Conventional polyester polyols are rooted in virgin raw materials and manufacture products through replicate esterification of diacids and glycols (e.g., succinic acid and 1,2-propanediol). These polyester polyols are easily distinguished by the structure of the monomers, molecular weight, and steric hindrance. Other polyester polyols originate from reclaimed starting materials and, thereby, produce low molecular weight aromatic polyester polyols that retain enough utility to be carried forward into other polymeric systems. Occasionally, polyols are blends of two or more polyols, each of specific molecular weights, to thereby provide intermediate molecular weight materials.
Polyols can be made, for example, by reacting epoxides (e.g., ethylene oxide) with an initiating molecule or agent, such as water. Such a process can efficiently make polyether diols like polyethylene glycol, polypropylene glycol, and poly(tetramethylene ether) glycol. Polyether polyols account for about 90% of the polymeric polyols used industrially with the remaining 10% being polyester polyols.
When polyols are reacted with a highly reactive poly-isocyanate, a polyurethane is produced. Polyurethanes are used to make many things including, for example but without limitation, automotive seats, elastomeric shoe soles, fibers (e.g., SPANDEX®, Invista S.a.r.l., Wichita, Kans.), adhesives, and foams used in, for example, insulation panels, seals, and gaskets.
Polyurethane (PU) polymers, since their inception, have proven to be diverse in structure and function. The production of polyurethanes from liquid diisocyanates and liquid polyether or polyester diols affords a variable motif when compared to other popular polymer systems. The step-by-step growth and synthesis afforded by polyurethanes provided a significant opportunity to build polymers with varied structures and properties. In 1952, polyisocyanates became commercially available, and commercial production of flexible PU foams began thereafter. Building on this technology, spray coating, reaction-in-molding, powder coating, and other techniques that use polyurethane polymers have greatly expanded over the past 60 years. Polyurethane polymers have shown their suitability for large surface area coatings and tank liners, and have demonstrated adhesion to concrete and steel, especially when coupled with a primer. Polyurethane polymers provide coatings that are durable, abrasion resistant, and corrosion resistant.
A polyurethane dispersion (PUD) is a free-flowing polyphasic system consisting of layers of water and polymer (e.g., a dispersed plastic). PUDs are often white translucent-to-opaque in appearance and are useful as coatings, film forming resins, and/or binders/adhesives, for example. The inherent lack of solvents in PUDs, coupled with ever increasing environmental demands, has aided their increased use and application, as well as their reputation as eco-friendly alternatives to more traditional organic solvent-based systems. The general advantages of PUDs are flexible at low temperatures, toughness, customizable mechanical properties, chemical resistance, and ability to be made hydrolytically stable. In order to impart many of these properties, it is necessary to create PUDs containing high molecular weight polymers. When the PUDs are dispersed, even those containing very high molecular weight polymers, their viscosities are determined only by the particle size (i.e., the volume fraction) in the dispersion.
Emulsions are often confused with PUDs, but emulsions result from a uniform particle size of a liquid media suspended within another immiscible liquid. In contrast, PUDs generally have a fairly broad distribution of different suspended particle sizes. Stable PUDs consist of spherical particles having a size in the range of from 30 nm to about 1,000 nm. Particles below 50 nm create a more transparent PUD, while PUDs containing particles above 1,000 nm produce a settleable solid fraction and a PUD having a very short shelf life. The contribution of the polymer solids to the total mass of the PUD is typically about 30-60%. Dispersions with high solid content have advantages in terms of transport and storage, ease of application, drying and cure times, all of which lead to a decrease of processing energy consumption. Furthermore, high solid content PUDs accentuate their environmental benefits and are becoming increasingly important.
Polyurethane densities are generally heavier than water thereby creating a tendency for the polymer to try to settle and coagulate. Coalescing forces are resisted by repulsion of the charged solubilizing groups on the particles and the attractive force that creates the systemic viscosity. In order to further combat such coagulation, PUDs may contain thickening agents and emulsifiers, which slow down the settling of the particles thereby improving shelf life. Moieties with non-ionic, cationic, and anionic hydrophilic groups can be incorporated into the polyurethane backbone or added as terminal groups in order to provide stabilization.
In addition to water, PUDs may contain hydrophilic organic solvents (e.g., N-methylpyrrolidone (NMP), glycol ethers, etc.). The addition of such a “co-solvent” enables the formation of hard polyurethane coatings by dissolving and softening the surface of the dispersed particles. After the water in the PUD is evaporated, a subsequently fused film is made (i.e., coalescence occurs). As a low vapor pressure solvent, the co-solvent evaporates gradually, allowing the film to become harder.
In order to ease the production of high molecular weight polyurethanes while preventing gelling, it is necessary to prepare these molecules maximally linear with a minimum of branching. With the materials of construction being simple bifunctional subunits, the shape, structure, and function of polyurethanes closely mirror the subunits from which the polyurethane is constructed. This creates an opportunity for the isocyanate to distinguish between aliphatic and aromatic polyurethane dispersions. Where aromatics are less expensive, they are known to yellow when exposed to light. Polyols often compose the largest mass fraction of PUDs and are generally seen as soft segments. Correspondingly, the glass transition temperature of the polyol is heavily influenced by the temperature flexibility profile.
Although PUDs generally contain the same components, the specific structures of each PUD may vary from product to product depending upon its specific structural components. As the chains are assembled, excess diisocyanate may be added, for example but without limitation, in order to provide terminal isocyanates that are further functionalized with difunctional molecules to interconnect the long-chain assemblies. The primary component of these chain assemblies is the ionic groups incorporated into the polymer to stabilize its water-dispersed particles. Dimethylol propioic acid (DMPA) results in, for example but without limitation, a polymer that is permanently hydrophilic and can be readily dispersed in an appropriate solvent system due to DMPA having carboxy and dihydroxy functionality allowing for its efficient incorporation into the backbone of the polymer while remaining functional as an ionic species. Similarly, cationic functionality can be added by combining quaternary amines such as N-methyl diethanolamine (NMDEA). Once the ionic groups (i.e., the cationic or anionic) are chosen, the particle size of the PUD can be controlled by the number of hydrophilic groups per given chain.
Typically, the preparation of a PUD in water requires a high shear force to obtain a correspondingly fine dispersion, as defined above. A common problem is the high viscosity of the undispersed isocyanate prepolymer. After chain extension, polyurethanes are practically not dispersible in water. In order to address such shortcomings, the prepolymer may be directly dispersed in water with high shear forces in the presence of the aqueous phase while heating in the presence of co-solvents. The heat may be applied to encourage dissolution or may be hot enough to melt the polyurethane into a liquid phase for dispersion. Alternatively, the co-solvent may be added directly to the solution, dispersed with water, and thereafter removed by distillation.
As previously stated, the use of PUDs are extensive. The resulting film can be dried at room temperature, or at elevated temperatures if required. After the water is evaporated, the gaps between each particle create high capillary forces that drive the particles to merge (i.e., coalescence) to form a homogeneous film. Co-solvents used to support the coalescence may remain for some time in the film after the water has evaporated. The co-solvent may also temporarily plasticize the coating and the resulting film may take some time to reach its final hardness.
Federal, state, and local regulations on the emissions of volatile organic compounds (VOCs) have pushed the use of PUDs into various industrial coatings markets including plastic, textile, leather, paper, and medical related products. These regulations have also catalyzed the expansion of PUDs into adhesives in, for example but without limitation, the shoe, automotive, and furniture industries. In many cases, these regulations have created an environment where PUDs are the preferred material because of their inherently low VOC content. Waterborne PUDs are, however, at a disadvantage as compared to solvent-based polyurethane solutions. It takes more time and/or energy to evaporate the water as compared to the VOCs used in solvent based polyurethane solutions. If one were just comparing the drying process alone, PUDs are disadvantaged when compared to solvent based polyurethane solutions. However, considering that greenhouse gases are emitted when organic solvents are used and the carbon footprint of solvent system is larger, i.e., many convert to carbon dioxide upon evaporation, the use of PUDs is becoming more routine.
As mentioned above, the growing concern about the hazardous and ecological impacts caused by the use of solvents, crosslinkers, and coalescing agents has led to the development of waterborne UV-curable polyurethane dispersions (UV-PUDs) and coatings. Ionic, isocyanate-terminated polyurethane prepolymers can be reacted with hydroxy-functional acrylates prior to dispersion into water in the presence of a neutralizing agent. Thereafter, the composition and size of the polymer chain is controlled to create the desired cross-linked content. As such, UV-PUDs are among the fastest growing coating type for wood products, for example.
Features of UV-PUDs, when compared with water/solvent free systems, retains the utility of both systems while not grossly suffering from the disadvantages of either. The UV-PUDs exhibit a delicate balance of both a strong chemical resistance (due to the UV-crosslinking) and enhanced toughness (due to the polyurethane character). The balance is struck from reduced cross-link density and thereby results in a feature set that very few coatings are able to offer: allowing for the UV-PUD to coalesce on a substrate and dry (through the loss of water) while also allowing the coating to be tack-free before curing. In this manner, such a coating affords the user the opportunity to further process the coated substrate (e.g., cutting, texturing, stamping, retreating, repairing, etc.) or even stack or roll the coated substrates for storage or shipment until subsequent UV treatment. Furthermore, balancing the crosslinking content to obtain good chemical resistance can be tipped to lower densities as polyurethane hard domains will provide additional chemical resistance. Componentry, such as mixed isocyanates and chain extenders, can also be added to the UV-PUDs in order to increase or change functionality of the UV-PUDs.
The presently disclosed and/or claimed inventive concept(s) relates to a sustainable PUD that incorporates a unique and novel blend or mixture of differing oligomeric polyols (i.e., dPET) obtained from polyethylene terephthalate that directly affects the performance characteristics of a resulting polyurethane film, adhesive, coating, and/or elastomeric material, and methods of producing the same. The presently disclosed and/or claimed inventive concept(s) also relates to the novel blend or mixture of differing oligomeric polyols and the methods of producing such.